I am expierencing low voltage on my 1991 Buick.

GM cars since the '87 models have had a very poor generator design that causes a lot of trouble. It is common to go through four to six of them in the life of the car, but you already did the best thing to reduce the number of those repeat failures. That is to replace the battery at the same time unless it is less than about two years old. In this case though, you have something else going on. These generators produce large voltage spikes that destroy the internal diodes and voltage regulator, (a good battery will dampen and absorb those spikes but they lose their ability to do that as they age), but those failures are almost always permanent.

You have an intermittent problem and any testing must be done while the problem is occurring. First of all look if the battery warning light turns on when the voltage drops. If it does, the voltage regulator knows there's a problem. If the light doesn't turn on, stop the engine, then turn the ignition switch back on. If the battery light does not turn on, there is a break in the wire going from the instrument cluster to it.

Restart the engine. If the warning light goes out but the gauge still shows low voltage, most likely the generator is working but the current isn't getting back to the battery. Measure the voltage at the battery and at the large bolted-on output terminal on the back of the generator. The two must be the same, and must be between 13.75 and 14.75 volts. If the generator voltage is higher, there is a break in that wire. Look for a large bolted-in fuse in the under-hood fuse box. Be sure those nuts are tight.

Another way to do the same thing is to stop the engine, then measure the voltage at the generator's output terminal. If you find 0 volts or something lower than the battery's voltage, there's a break in that wire or that fuse has a bad connection.

It sure sounds like your new generator is defective. It could have one bad diode of the six. That will reduce its output current capacity to exactly one third of its rating. If it can only produce around 30 amps, that is not enough to run all the stuff on the car like the fuel pump, heater fan, lights, ignition system, and all the computers. The battery will have to make up the difference but the symptom would be the voltage is gradually falling. It isn't likely it would suddenly pop back up unless a diode has a bad connection. Shorted diodes don't suddenly get better.

The internal voltage regulator is another prime suspect but they too generally fail, then they're done for. They are rarely intermittent.

I would start with a load test but they would have to catch it while the problem is occurring. That will show if it can deliver the full typical 90 or more amps, and it will show "ripple". With one bad diode ripple will be very high and maximum output current will be real low. If the output is 0 amps, the voltage regulator is quitting intermittently.